The invention relates to an optical information memory apparatus and its control method and, more particularly, to an optical information memory apparatus and its control method for improving the reliability of a seeking system upon fine seeking of an optical information memory apparatus of the two-stage seeking system type.
In an optical disc apparatus, as one of the seeking systems for moving a light spot for recording and reproducing on and from a target track, the two-stage seeking system comprising a combination of a coarse seeking system and a fine seeking system has been known.
As examples of the above two-stage seeking system, there have been known systems disclosed in JP-A-58-169370 and JP-A-58-91536.
According to the above seeking system, an optical head is first moved to a position near a target track at a high speed by using a coarse actuator, a light spot is settled to a proper track near the target track to be sought, a jumping operation is subsequently repeated track-by-track a fine actuator such as a galvano mirror or the like mounted onto the optical head, and the light spot is finally positioned to the target track.
According to the two-stage seeking system, a coarse actuator comprising an optical linear scale (a scale pitch is an integer times as large as a track pitch) is used as a position detector of the optical head and the optical head is positioned (coarse seeking) to a location near the target track at a high speed by an output of the linear scale. After that, the light spot is brought to a track near the target track after waiting for a state in which the light spot is settled, that is, a decentering speed of the track for the light spot (that is, an oscillating speed when the track is relatively oscillated in the disc radial direction in a sine manner for the light spot due to an eccentricity of the track of an optical disc by using one rotation of the optical disc as a period) is sufficiently delayed, a track address is read, and the jumping operation of the light spot track-by-track is subsequently executed, thereby moving (fine seeking) the light spot to the target track.
The above conventional two-stage seeking system will now be described with reference to FIG. 1, showing an operation sequence, and FIGS. 2 and 3, showing arrangement diagrams of pits on an optical disc.
As shown in FIG. 1, when a coarse seeking instruction signal 5 is first turned on, the optical head starts to move toward a target address to be sought. In the above case, a movement distance of the optical head is calculated by a linear scale position signal 2. A moving speed of the optical head is detected by differentiating the signal 2. A speed control of the optical head is executed on the basis of the results of the movement distance and the moving speed.
When the optical head moves to a position near a target point in a track detecting accuracy range of the linear scale and the moving speed becomes sufficiently small, the coarse seeking instruction signal 5 is turned off and the speed control of the optical head is finished. At the same time, a coarse positioning control instruction signal 6 is turned on and the optical head is coarsely positioned by the linear scale position signal 2. At that time, in order to detect that a track decentering speed for the light spot has decreased to a predetermined value or less just after the start of the coarse positioning control, a tracking error zero-cross pulse 4 whose level is inverted each time a tracking error signal 3 zero-crosses (namely, each time the light spot transverses the track) is generated.
A continuation time T of a low- or high-level state of the pulse 4 is monitored. If it is detected that the continuation time T has exceeded a predetermined time T.sub.0 by a state in which the decentering speed had been reduced to a small enough value, the coarse positioning control instruction signal 6 is turned off and a tracking control instruction signal 7 is simultaneously turned on. To read a track address near the target point to be sought at that time, the operation for allowing the light spot to once follow the center of the track near the target point to be sought is started. After that, the light spot is jumped every track as mentioned above, thereby executing the fine seeking operation to move the light spot to the target track to be sought.
According to the above conventional two-stage seeking system, it is unclear at which position in the radial direction for the track center near the seeking target point the light spot exists at a time point when the tracking control instruction signal 7 is turned on.
As shown in FIG. 2, now assuming that a recording pit 8 is located symmetrically with respect to the right and left directions for the track center, and the tracking error signal 3 (for a deviation state of the light spot to the track center, since a state of the reflected light including irregular reflections by grooves on both sides of the track becomes unbalanced in accordance with the deviation amount, such a deviation amount can be detected by a pair of right and left detectors for detecting such an unbalanced state) is symmetrical with respect to the positive and negative polarities and no offset occurs, even if the tracking control operation has been turned on at a point A which is fairly distant from the track center, the light spot is subjected to the acceleration which is proportional to a magnitude of the tracking error signal 3 from the point A, so that the light spot is accelerated toward a target point B serving as a track center and passes the target point B. After the light spot passes the point B, the polarity of the tracking error signal 3 is changed to the negative polarity, so that the light spot is subjected to the acceleration in the direction opposite to the moving direction and is returned to the target point B. Finally, the light spot is settled to the target point B. A desired track pull-in is thus accomplished.
As shown in FIG. 3, in the case where the recording pit 8 is deviated in the radial direction of the optical disc from the track center and is located, amplitudes Pu and PL in the positive and negative directions of the tracking error signal 3 are asymmetrical with respect to the positive and negative polarities (Pu&gt;PL). This is because the tracking error signal shown in FIG. 2 is further deviated to either one of the positive and negative directions by an influence such that when the recording pit is deviated from the track center, a groove between tracks is destroyed as shown at 8a in FIG. 3, so that the grooves on both sides of the track become asymmetrical with respect to the right and left positions or the like. In the case where the tracking error signal 3 is asymmetrical with respect to the positive and negative polarities as mentioned above, when the tracking control operation is turned on at a point C which is fairly away from the track center, the light spot is subjected to the acceleration which is proportional to the positive amplitude Pu of the tracking error signal 3 and is accelerated. After the light spot passes the peak in the positive direction, it passes the target point B.
After the light spot passes the target point B, the light spot is subjected to the acceleration such as to be returned in the direction of the target point B due to the negative polarity of the tracking error signal 3. However, the amplitude PL of the valley in the negative direction is smaller than the amplitude Pu of the peak in the positive direction because of the offset in the positive/negative directions of the tracking error signal 3, so that kinetic energy for allowing the light spot to be away from the target point B is larger than the energy to return the light spot toward the target point B. Consequently, the light spot is also continuously accelerated in the same direction by the offset after that, so that the pull-in for the tracking control cannot be performed.